I wrote this r&r diary entry yesterday about how a band should always be moving forward, evolving, growing, morphing.
Is it true? I think so. You want your r&r bands to be adventurous, to try different things. There are exceptions. Some bands are just good at one thing. No need to alter the formula. I think of The Ramones as example one.
But I think the creative impulse leads to experimentation. Or at least if you are open and true to the impulse. Keep it real. Keep it live. Keep it moving.